1. Field of the Invention
The present invention relates to a method and apparatus for measuring the distance to a target to be detected in the ground, water, snow or the like, the slag level or molten steel level in a melting reduction furnace, converter or the like and the distance to an ordinary target to be detected.
2. Description of the Prior Art
Methods heretofore known or conceived for measuring the distance to a target to be detected in the ground or water in a noncontact manner by using, for example, an electromagnetic wave include one which transmits a monopulse of the order of several ns(10.sup.-9 seconds) as shown, for example, in Japanese Patent Publication No. 55-44916 or another which transmits a pseudo random signal instead of a pulse signal as shown for example, in a publication "Subsurface Radar Using Code Pulse" (Suzuki et al. Institute of Electronics, Information and Communication Engineers, Technical Report SANE 87-1, 1987).
In the method of transmitting a pseudo random signal, such a pseudo random signal as an M-sequence (maximal length sequence) signal or Barker code signal tending to easily produce an autocorrelation output is generated with a given PRF (pulse repetition frequency) so that after its power amplification, the generated signal is transmitted as an electromagnetic wave into the ground or water through a transmitting antenna. The reflected wave from the target to be detected is received through a receiving antenna so that after the conversion from the high-speed received signal to a low-speed received signal by a sampling device, the received output is subjected to pulse compression by a correlator and a detected signal increased in signal amplitude is generated. The time interval between the transmission of the pseudo random signal and the generation of the detected signal from the correlator is equal to the propagation time of the electromagnetic wave going back and forth the distance between the transmitting and receiving antennas and the target to be detected as in the case of the ordinary radar and therefore the distance to the target to be detected can be calculated. Generally, the electromagnetic wave is attenuated more under the ground than in the space and its phase is also changed. Thus, the frequency of the electromagnetic wave used is selected on the basis of the properties of the earth (e.g., the wet soil or dry soil), the detecting distance, etc.
The above-described conventional method and apparatus of the type designed to measure the time between the transmission of a pseudo random signal and the generation of a detected signal from the correlator has the disadvantage of requiring a sampling device in the apparatus, thereby increasing the number of component parts, the scale of the apparatus and the cost. In addition, the correlator requires a tapped delay line as its component part, with the resulting disadvantage of causing a measuring error by a waveform distortion caused by the passing of the received signal through the delay line.
On the other hand, a method of forming a correlator by subjecting the received signal to A-D conversion and performing a digital signal processing has not been put in practice as an economical apparatus due to the limited response speeds of circuit elements and the difficulty of the real time processing.
Also, the conventional level measuring methods for the slag level, molten steel level, etc., in melting reduction furnaces, converters, etc., and position measuring methods for targets to be detected have been divided roughly into two types, i.e., contact type and non-contact type.
The contact-type methods include an electric conduction detection type and an apparatus of this type has been devised as shown, for example, in Japanese Laid-Open Utility Model No. 61-129858 in which at least two electrodes are moved up and down from the top of a furnace and a voltage is applied between the electrodes, thereby detecting the presence of slag by an electric conduction between the electrodes and measuring the slag level by the position of the electrodes.
Also, for performing a temperature distribution measuring method, which is one of the contact type methods, an apparatus has been proposed in which, as shown, for example, in Japanese Laid-Open Patent No. 61-217516, a large number of temperature sensors are embedded at suitable intervals in the lance of a converter and the temperature distribution in the furnace is continuously measured by the temperature sensors, thereby measuring the slag level on the basis of the characteristics of the temperature distribution.
The non-contact type methods include a microwave FMCW (frequency modulated continuous wave) method which has been proposed, for example in Japanese Laid-Open Patent No. 63-21584 or 61-57875 in which a continuous microwave having a frequency of about 10 GHz is frequency modulated and transmitted from an antenna toward a surface to be measured, whereby the beat frequency resulting from the mixing of the transmitting signal and the reflected wave from the surface to be measured is counted and the level of the surface is measured. This method measures the distance from the antenna to the target to be measured on the ground that the required propagation time for the microwave to go back and forth the distance between the antenna and the target to be measured corresponds to the beat frequency.
Also, as a microwave pulse modulation type, there has been known a method in which, as in the case of the ordinary radar to detect a flying target, a microwave having a frequency of about 10 to 20 GHz is pulse modulated and transmitted to measure the distance to a target to be detected on the basis of the fact that the required wave propagation time until the reception of the reflected wave from the target to be detected is proportional to the distance to the target.
In addition, as shown, for example, in a publication "An Experimental Study of a Pulse Compression Radar Using Random Series" (Nishimoto et al. The Institute of Electronics, Information and Communication Engineers, Technical Report SANE 85-25, September 1985), a measuring method employing a radar has been known in which a carrier wave having a frequency of 1 to several tens GHz is modulated by a pseudo random signal, e.g., a maximal length sequence signal, and transmitted to a target to receive the reflected wave from the target, and an optimal matched filter combining a tapped delay line and a weighted adder is used in a demodulation system effect a pulse compression and thereby to improve the resolution and sensitivity.
With the conventional level measuring methods above-mentioned or the methods for measuring the distance to a target, if the methods are of the contact type, they all tend to deteriorate the durability of the portions which contact the slag or molten steel in a furnace or tend to cause damages to those portions. In the case of the electric detection type of conduction, there are disadvantages that an erroneous signal is generated due to an insulation failure of the electric insulating portion caused by the dust or the molten steel splash in a furnace, that a continuous measurement cannot be effected due to the detection by the vertical movement of the electrodes and so on.
Also, in the case of the temperature distribution measuring method of the contact type, the embedding of the temperature sensors in the lance cooled with water deteriorates the response of the temperature sensors due to the heat-transfer characteristic of the lance. There is another disadvantage that when increasing the number of temperature sensors for the purpose of improving the measuring accuracy, there are many restrictions to the wiring of the sensors from the spatial and temperature point of view.
In the case of the microwave FMCW method of the non-contact type, since the inside space of the furnace is limited and the wave reflectors such as the lance and the charging hole of the furnace are present within the space, when a microwave is transmitted into the furnace, undesired reflected waves, including multipath reflected waves, are generated with the resulting disadvantage of making it difficult to eliminate the undesired reflected signals and thereby accurately measure only the reflected signal from the intended target to be detected.
Also, in the case of the microwave pulse modulation method of the non-contact type, there are disadvantages. For example, since usually the signal reception is effected after the transmission of the pulse modulated wave has been completed, considering the propagation velocity of the microwave, it is necessary for a short distance measuring radar to transmit a microwave having a relatively large peak transmission power and modulated by a pulse of a very short time width and to measure a small time required until the reception of a received signal reflected from a short distance target and it is difficult to technically realize these operations. This makes the method unsuitable for use in short distance measurements such as the level measurement in a furnace and so on.
Further, in the case of the microwave pulse compression radar method of the non-contact type, there are disadvantages that while, after receipt the signal, the pulse width is compressed increase the received power and thereby to improve the resolution and sensitivity, the construction of the optimal matched filter combining the tapped delay line and the weighted adder required in the demodulation system is complicated. In particular, if a pseudo random signal length is increased (e.g., 2.sup.5 to 2.sup.20) so as to enhance the sensitivity, the apparatus is complicated in construction and increased in size, thereby increasing the cost. There is another disadvantage of requiring complicated operations such as adjustments of the delay times among the taps and adjustment for the correction of a waveform distortion during the propagation in the delay line. Still another disadvantage is that where the function of the matched filter is performed by a digital signal processing, a high-speed A-D converter and a high-speed computing unit are required, thereby similarly complicating the construction of the apparatus, increasing its size and increasing the cost.